1. Field of the Invention
The present invention relates to a relay station, a base station, a power management method, and a computer readable medium thereof for use in a wireless mesh network. More specifically, the present invention relates to a relay station, a base station, a power management method, and a computer readable medium thereof for use in a wireless mesh network based on the IEEE 802.11 standard.
2. Descriptions of the Related Art
Over recent years, an emerging network architecture known as the wireless mesh network has become more popular. Accordingly, numerous companies, such as Motorola, have begun to develop the associated software and hardware facilities for the wireless mesh network. FIG. 1 illustrates a schematic view of a conventional wireless mesh network 1. The wireless mesh network 1 is primarily composed of an infrastructure wireless local area network (WLAN) 10 connected with an ad-hoc network 11 via a relay station (RS) 120, i.e., a mobile station (MS) with a relaying capability.
The infrastructure WLAN 10 comprises a base station (BS) 100, mobile stations (MSs) 101, 102 and a relay station 120. From the viewpoint of the infrastructure WLAN 10, the relay station 120 does not perform any relaying operations and thus is considered simply as an MS. Each of the MSs (i.e., the MSs 101, 102 and the RS 120) within the infrastructure WLAN 10 must be registered with the BS 100 to share internet services. More particularly, at the start of a beacon interval, the BS 100 broadcasts a beacon to each of the MSs (i.e., the MSs 101, 102 and the RS 120) within the infrastructure WLAN 10. There are two main functions of the beacon. One of the functions is for all MSs (i.e., the MSs 101, 102 and the RS 120) to synchronize with BS 100. The other function is for the BS to inform all MSs in power saving mode whether they have data buffered in the BS 100. Each of the MSs in power saving mode checks if the BS 100 attempts to transmit data to the MS according to the traffic indication map (TIM) in the beacon. The MSs that does not have data buffered in the BS 100 may remain in power saving mode. On the other hand, the MSs which need to receive data from the BS 100 needs to leave the power saving mode and transmit a power saving poll to the BS 100 to inform the BS 100 to transmit the data now. In this way, various internet services are enabled to run smoothly.
In addition, the ad-hoc network 11 comprises MSs 110, 111 and the RS 120. From the viewpoint of the ad-hoc network 11, the RS 120 does not perform any relaying operations and thus is simply considered as an MS. In the power-saving mode, each beacon interval is divided into an Announcement Traffic Indication Map window (ATIM window) and a data window. At the start of the beacon interval, each of the MSs (i.e., the MSs 110, 111 and the RS 120) within the ad-hoc network 11 wakes up to compete sending a beacon, and the winner gets the right to send a beacon, wherein the beacon is used to accomplish the synchronization with the other MSs. When attempting to transmit data to the RS 120, the MS 110 sends an Announcement Traffic Indication Map frame (ATIM frame) during the ATIM window to inform the RS 120 of this attempt. In response to the ATIM frame, the RS 120 returns an ATIM ACK to the MS 110. Within the data window, the MS 110 and the RS 120 keep awake all along, so that the MS 110 can transmit data to the RS 120. MSs (i.e., the MSs 110, 111 or the RS 120) that neither transmitted nor received an ATIM frame may return to the doze state at the end of ATIM window.
The wireless mesh network 1 combines the infrastructure WLAN 10 and the ad-hoc H network 11 as shown in FIG. 1. The advantage of the wireless mesh network 1 is that the MSs (e.g., the MS 110 or 111) still enjoy the Internet services by connecting with the BS 100 via the RS 120 even the MSs are not located within the coverage area of the BS 100.
However, when acting in power saving mode, the wireless mesh network 1 has the following disadvantages. FIG. 2 illustrates a schematic view of a signal transmission in the conventional wireless mesh network 1. In FIG. 2, the axes corresponding to the BS, the RS and the MS denote time axes corresponding to the BS 100, the RS 120, and the MS 110 of FIG. 1 respectively. The beacon interval 2 includes an ATIM window 20 and a data window 21. Since the BS 100 and the RS 120 belong to the infrastructure WLAN, the BS 100 is unaware of the ATIM window 20 defined in the beacon interval as well as the presence of the MS 110.
In the example, the BS 100 attempts to transmit data 204 to the RS 120 and the RS 120 in turn to transmit the data 204 to the MS 110. At the outset, the BS 100 broadcasts a beacon 200 to the RS 120. Since the BS 100 and the MS 110 are not located within the coverage areas of each other, neither of them can receive the beacon transmitted by the other. By checking the beacon 200, the RS 120 learns that the BS 100 attempts to transmit the data 204, and then sends a power saving poll 202 to the BS 100. In response to the power saving poll 202, the BS 100 proceeds to transmit the data 204. After receiving the data 204, the RS 120 sends a data acknowledge signal 205 to the BS 100. At this point, a problem occurs. Because the size of data 204 may be large, the RS 120 probably has no chance to send an ATIM frame to the MS 110 prior to the end of the ATIM window 20, causing the MS 110 to return to doze state when the ATIM window 20 comes to an end. Consequently, the RS 120 has to wait until the next beacon interval to retry transmission of the data 204 to the MS 110. In other words, the data 204 experiences a delay longer than one beacon interval. Moreover, from the viewpoint of the RS 120 and the MS 110 (i.e., the ad-hoc network 11), the ATIM window 20 is used to transmit control signals (including an ATIM frame) for the individual MSs rather than to transmit data. The data transmission should occur within the data window.
FIG. 3 illustrates another schematic view of a signal transmission in the wireless mesh network 1. Because FIG. 3 is similar to FIG. 2, only the different portions will be described herein. In FIG. 3, both the BS 100 and the MS 110 attempt to transmit data to the RS 120. The MS 110, which is attempting to transmit data to the RS 120, has to send an ATIM frame 206 to the RS 120. However, in this case, the RS 120 also receives data 204 simultaneously while receiving the ATIM frame 206, thus leading to a collision 207. This makes it impossible for the RS 120 to tell whether the receiving frame is the ATIM frame 206 from the MS 110 or the data 204 from the BS 100. As a consequence, both the data 204 and the ATIM frame 206 of the MS 110 have to be retransmitted.
Both cases illustrated in FIG. 2 and FIG. 3 may cause a decrease in the throughput of the wireless mesh network 1 within a single beacon interval. Therefore, it is important to increase the throughput within a single beacon interval by delaying data transmission from the ATIM window to the data window.